Calculating the CFM for Carburators
Thread Starter
Drifting
Joined: Jul 2002
Posts: 1,254
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From: Edinburgh
Calculating the CFM for Carburators
I read somewhere that to calculate the CFM for your carb you multiply engine size by max revs, divided by 3456. That figure is then factored for efficiency and that would be 75% for a standard engine and about 85% for good heads headers etc. So if I convert that to numbers I get 350x6000/3456 = 608.Then factor by85% = 516CFM.
All you guys with road tuned engines seem to use 650CFM carbs are my calcs wrong? :confused:
All you guys with road tuned engines seem to use 650CFM carbs are my calcs wrong? :confused:
Drifting
Joined: Jul 2002
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From: Cocoa Beach FL
Re: Calculating the CFM for Carburators (Graemeinvette)
I found this calculator on...another vette forum :lol:
http://www.corvetteactioncenter.com/...ools/carb.html
http://www.corvetteactioncenter.com/...ools/carb.html
Thread Starter
Drifting
Joined: Jul 2002
Posts: 1,254
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From: Edinburgh
Re: Calculating the CFM for Carburators (Mystery Machine)
That seems to put it a bit higher as a minimum and takes it to 670 as a max. Which puts a 650CFM carb in the correct ballpark.
Thanks for that :cheers: :cheers:
Thanks for that :cheers: :cheers:
Drifting
Joined: Jul 2002
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From: Cocoa Beach FL
Re: Calculating the CFM for Carburators (Graemeinvette)
I was in the same quandry a few days ago. I had always thought I was running a Performer 600cfm (Edelbrock) which I figured might be a bit small but I figured I wasn't gonna doll out 300-400 clams for 50 more cfm. Then I got the brainiac idea to check the p/n of my carb, come to find otu it's only a 500cfm :eek: Needless to say a new Demon 650cfm should be on my doorstep this afternoon :thumbs:
Thread Starter
Drifting
Joined: Jul 2002
Posts: 1,254
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From: Edinburgh
Re: Calculating the CFM for Carburators (Mystery Machine)
Yeah I was doing the maths prior to ordering a Demon, after reading all the posts here it seems to be the way to go. Not sure what to do about the electric choke though, since mine is currently manual. Any thoughts? :confused:
Drifting
Joined: Jul 2002
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From: Cocoa Beach FL
Re: Calculating the CFM for Carburators (Graemeinvette)
I talked to a bunch of people before deciding on the Demon but after one firend pointed this out:
http://www.demoncarbs.com/demoncarbs/demon_charts.htm
and I talked to Lars who mentioned that the Demons he was playing with were producing somewhere around 15-20 additional HP on the dyno I decided it was the better route to go.
As far as the choke issue...I have no idea how I'm gonna address that one. I usually end up flying by the seat of my pants with stuff like this...you know throw the directions away first thing! Besides I NEVER use the choke anyway so hopefully it on'e be an issue. We'll see. ;)
http://www.demoncarbs.com/demoncarbs/demon_charts.htm
and I talked to Lars who mentioned that the Demons he was playing with were producing somewhere around 15-20 additional HP on the dyno I decided it was the better route to go.
As far as the choke issue...I have no idea how I'm gonna address that one. I usually end up flying by the seat of my pants with stuff like this...you know throw the directions away first thing! Besides I NEVER use the choke anyway so hopefully it on'e be an issue. We'll see. ;)
Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
Re: Calculating the CFM for Carburators (Graemeinvette)
CARBS Tests TQ/HP http://www.corvettefaq.com/c3/carbcomparo.htm
You need to give your setup to determine cfm needed. Normally the formulas are really for single plane intakes.
For all who base their buying decisions on info. by the seller, do not buy oceanfront property in Arizona even if there are no beach restriction laws!
You need to give your setup to determine cfm needed. Normally the formulas are really for single plane intakes.
For all who base their buying decisions on info. by the seller, do not buy oceanfront property in Arizona even if there are no beach restriction laws!
Thread Starter
Drifting
Joined: Jul 2002
Posts: 1,254
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From: Edinburgh
Re: Calculating the CFM for Carburators (Ganey)
Ganey,
You have obviously done a fair bit of research on this. A 650 Demon is what I am going to get, my inlet manifold appears to be stock. What inlet manifold should I get, bearing in mind that I want it to fit under the std small block hood?
You have obviously done a fair bit of research on this. A 650 Demon is what I am going to get, my inlet manifold appears to be stock. What inlet manifold should I get, bearing in mind that I want it to fit under the std small block hood?
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Drifting
Joined: Jul 2002
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From: Cocoa Beach FL
Re: Calculating the CFM for Carburators (Ganey)
For all who base their buying decisions on info. by the seller, do not buy oceanfront property in Arizona even if there are no beach restriction laws!
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
Re: Calculating the CFM for Carburators (Graemeinvette)
You probably want to get Weiand Action+ or Edel. EPS. Stuffing a RPM under there at any cost & restricting intake is not a great idea as one would consider the entire flow path & affect on the carb. RPM fits fine on med. rise 73-79 hoods.
Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Re: Calculating the CFM for Carburators (Graemeinvette)
http://www.prestage.com has many different calculators for all kinds of engine related things. You can find it all under the resources car math menu on the left.
Thread Starter
Drifting
Joined: Jul 2002
Posts: 1,254
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From: Edinburgh
Re: Calculating the CFM for Carburators (Sigforty)
Yup , that site gives me 516 CFM same as I calculated. But since its hardly an exact science I will go with the 650CFM Demon. :cheers: :cheers:
Thanks guys interesting thread :thumbs:
Thanks guys interesting thread :thumbs:
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Calculating the CFM for Carburators (Graemeinvette)
The best formula I have ever seen is 1.6 CFM per hp. I only own Demon mechanical secondary carbs. If I had it to do over again I would buy a 750 Vacuum secondary Speed Demon.
The only real test is to hook up a carb and go out and drive it with a vacuum guage attached. Your inches of vacuum will increase at WOT as your rpm's increase. I don't remember the exact desired max inches of vacuum at redline. But lets say that it rises to 10 inches with a 600 cfm carb. Then you install a 850 cfm and the max inches of vacuum might drop to 5 inches. You motor will achive max HP with @ a max of 5 inches. But that figure doesn't address drivability
More and more winners at bracket racing are showing up with Vacuum S. carbs. You can't beat consistancy and the off idle responce.
The only real test is to hook up a carb and go out and drive it with a vacuum guage attached. Your inches of vacuum will increase at WOT as your rpm's increase. I don't remember the exact desired max inches of vacuum at redline. But lets say that it rises to 10 inches with a 600 cfm carb. Then you install a 850 cfm and the max inches of vacuum might drop to 5 inches. You motor will achive max HP with @ a max of 5 inches. But that figure doesn't address drivability
More and more winners at bracket racing are showing up with Vacuum S. carbs. You can't beat consistancy and the off idle responce.
Drifting
Joined: Jul 2002
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From: Cocoa Beach FL
Re: Calculating the CFM for Carburators (Graemeinvette)
Thanks guys interesting thread :thumbs:
Drifting
Joined: Jan 2002
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From: Northampton Ma
Re: Calculating the CFM for Carburators (Mystery Machine)
I'll be ordering a 750 Speed Demon, mechanical secondaries, no choke carb for my new crate motor. I talked it over with Lars, and he said this would be the best for my application (470hp, 383ci). He said he had dynoed many engines with Holley's and Demon's, with all mixes of secondaries and chokes and said this one performed the best. Apparently, he said the springs in the vac. secondary Demon were to stiff and didn't open fully at WOT, even when they replaced the springs with softer ones, it still couldn't keep up with the mechanical secondaries :cheers: As for hooking up an electric choke, you'll have to tap into a 12 volt source, the "hot" alternator wire, or the yellow wiper motor wire are a good place for that.
[Modified by Ryan77, 4:36 PM 2/13/2003]
[Modified by Ryan77, 4:36 PM 2/13/2003]
Melting Slicks
Joined: Mar 1999
Posts: 2,845
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From: Apex NC
Re: Calculating the CFM for Carburators (Ryan77)
I have to wonder about all the quoted HP gains from these carbs. Are the HP gains across the entire RPM range, or only at the top end? What about low-rpm torque? Do some of the "high performance" solutions actually rob you of torque lower in the powerband? For a typical street driven car, don't you want to maximize torque throughout the RPM range? If a carb swap gains you +20hp at 6500 rpm but you lose 20 ft-lbs of torque at 2000 rpm, is it truly a net gain when it comes to driveability?
edit - looking at the data on the Demon site shows an across-the-board increase in torque. Dunno about other dyno results from Lars' tests. Anyone?
[Modified by sb69coupe, 4:58 PM 2/13/2003]
edit - looking at the data on the Demon site shows an across-the-board increase in torque. Dunno about other dyno results from Lars' tests. Anyone?
[Modified by sb69coupe, 4:58 PM 2/13/2003]
Team Owner
Joined: Apr 1999
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Calculating the CFM for Carburators (Ryan77)
Ryan77 Both of my Demons are the chokeless variety. I never have a problem starting. Just a couple of pumps on the gas petal and hit the starter. I do have an 1/8 inch hole drilled in each primary blade to lean out the idle and the squirters are drilled out to .044 for a faster big shot of gas. My 825 cfm Race demon was a bit much for my 383. So I went back down to 750 Speed demon.
